The present invention relates to a process of forming a laminated piezoelectric structure, and to piezoelectric transducers, particularly hydrophones, employing the laminate.
Piezoelectric polymer films of polyvinylidene fluoride (PVDF) have been used as sensing materials in a variety of applications. These films also possess pyroelectric activity. The principal crystalline forms of PVDF are the highly polar beta form and the non-polar alpha form. High piezoelectric and pyroelectric activities are associated with the polar beta form. In order to increase both the piezoelectric and pyroelectric activities of PVDF, the film is mechanically oriented and subjected to an intense electrical field, otherwise known as poling, to cause the oriented beta form crystallites to predominate.
Piezoelectric polyvinylidene fluoride film is a particularly useful sensing material for hydrophone applications because its acoustic impedance is close to that of water, and it possesses a hydrostatic stress constant (g.sub.h) which is greater than that of conventional ceramic piezoelectric materials, such as barium titanate or lead zirconate titanate (PZT). It is also generally known that the sensitivity of such films for acoustic applications generally increases with the thickness of the piezoelectric film. Unfortunately, the poling step places a limit on the maximum thickness of fully polarized polyvinylidene fluoride films.
R. M. Quilliam in U.S. Pat. No. 4,405,401 issued Sep. 20, 1983 discloses that instead of using of a single thick polymeric piezoelectric film, a piezoelectric/pyroelectric element can be formed from a plurality of thin piezoelectric films. A laminated stack is formed by first individually coating the mechanically oriented, unpolarized PVDF films with a nitrile rubber-based adhesive. The adhesive is allowed to dry, and the films are assembled into a stack between two plates of a press. The platens of the press are heated and pressure is applied to reactivate the glue to form a bonded stack. A voltage is then applied across the heated stack to polarize the PVDF films. Quilliam also discloses that two or more of the polarized stacks may be assembled together with an inner electrode positioned between adjacent polarized stacks.
The glued laminated structure of Quilliam provides for piezoelectric elements with greater sensitivity than single layer devices. However, the dissimilar glue layer forms an interface with the piezoelectric film where some of the acoustic waves are reflected. This results in a decrease in the acoustic sensitivity of the device. The present invention overcomes this problem by laminating the piezoelectric layers so that the interface of dissimilar materials is eliminated.